1. Field of the Invention
Exemplary aspects of the present invention relate to a light source device having: a light-emitting tube including a light-emitting portion that generates a light beam by an electric discharge between electrodes and a sealing portion provided on both sides of the light-emitting portion; an ellipsoidal reflector having a substantially ellipsoidal reflecting surface and irradiating a light beam irradiated by the light-emitting tube after converging at a predetermined position; a sub-reflection mirror having a reflecting surface being opposed to the reflecting surface of the ellipsoidal reflector and covering a front side of the light-emitting tube in the light-irradiation direction to reflect the light beam emitted by the light-emitting tube toward the ellipsoidal reflector; and a transparent member provided in front of the ellipsoidal reflector in the light-irradiation direction to transmit the light beam, and a projector equipped with the light source device.
2. Description of Related Art
Conventionally, projectors that modulate a light beam irradiated by a light source in accordance with image information to project an optical image in an enlarged manner have been used for presentation purpose at conferences, academic conferences, exhibitions etc. with a personal computer, and recently, such projectors are also used for a home theater.
A related art light source device used for such projector typically includes a discharge light-emitting tube with high intensity such as a metal halide lamp and a high pressure mercury lamp with a reflector attached thereto. Recently, there has been known a light source device employing an ellipsoidal reflector having an ellipsoidal reflecting surface (see, for example, Reference: Japanese Patent Laid-Open Publication No. 2000-347293, FIGS. 1 and 3).
In the ellipsoidal reflector, a light-emitting center of the light-emitting tube is located at a position of an ellipsoidal first focus on the rear side of the ellipsoidal reflector in the light-irradiating direction thereof so that a reflected light beam is converged at a position of a second focus on the front side in the light-irradiation direction. In the light source device having a sub-reflection mirror as well as the ellipsoidal reflector, the first focus and the second focus of the ellipsoidal reflector can be positioned closer to each other and the opening diameter can be smaller or shorter relative to the amount of the condensed light, so that the light can be efficiently condensed even with a light source with low output. Therefore, the projector with such light source device incorporated therein can be downsized and intensity thereof can be enhanced.
On the other hand, as disclosed in the Reference, a transparent member such as a parallelizing lens is provided in front of the ellipsoidal reflector in the light-irradiation direction, and a source light is incident on an optical system located at the downstream of the light source device through the transparent member.
It is preferable that the transparent member is held on a predetermined optical axis by a holder, and that the holder is provided so as to cover a front end of the ellipsoidal reflector in the light-irradiation direction over an outer periphery of the transparent member to prevent leakage of a stray light as an unused light that cannot be optically controlled.
Here, the holder likely faces thermal deterioration caused by irradiation of the light beam irradiated by the light-emitting tube. Thus, a shielding member using a metal material such as aluminum is provided to the holder to enhance thermal resistance.
When the short ellipsoidal reflector and sub-reflection mirror as described above are employed, the sub-reflection mirror is preferably configured to transmit an infrared ray or an ultraviolet ray to prevent overheating of the light-emitting tube covered by the sub-reflection mirror. In such case, since the infrared ray or ultraviolet ray is transmitted through a reflecting surface of the sub-reflection mirror toward the position where the transparent member is located while a visible light is reflected by the sub-reflection mirror toward the ellipsoidal reflector, the holder holding the transparent member is illuminated by the infrared ray or ultraviolet ray. Since the light-emitting tube is protruded toward the front side of the ellipsoidal reflector in the light-irradiation direction, the light-emitting tube is opposed to an inner circumferential surface of the holder. In the above configuration, since the stray light of the infrared ray or the ultraviolet ray transmitted through the sub-reflection mirror is reflected by the metallic shielding member provided at the inner side of the holder, a sealing portion or a light-emitting portion of the light-emitting tube is irradiated by the reflected stray light, which results in shortening lifetime of the light-emitting tube caused by overheating. Further, temperature rise inside of the light source device has also been a problem.
Also, in addition to the reflection of the stray light at the holder, a tip end of the sealing portion at the front side of the light-emitting tube in the light-irradiation direction is close to the light converging point as the second focus point of the ellipsoidal reflector, and thus faces harsh temperature rise, so that temperature of this portion requires to be lowered.
On the other hand, when the light-emitting tube is cooled with the air sent by a fan or the like, since a big opening cannot be formed at the holder shielding the stray light, sufficient cooling should be difficult. In addition, increase in size or rotation speed of the fan will cause generation of noise.